Method of making an electroluminescent phosphor



United States PatentC METHOD OF MAKING AN ELECTROLUMI- NESCENT PHOSPHORKeith H. Butler, Marblehead, and Richard M. Rulon, Salem, Mass.,assignors to Sylvania Electric Products, Inc., Salem, Mass., acorporation of Massachusetts No Drawing. Application July 22, 1952,Serial No. 300,358

2 Claims. (Cl. 252-3015 This invention relates to electroluminescentphosphors and to methods of manufacture thereof.

Certain phosphors of this type are shown in a copending applicationSerial No. 230,711, filed June 8, 1951, by Keith H. Butler. Zincsulphide activated with copper, and with manganese in addition ifdesired, and containing a small quantity of lead, is especiallyeffective as such a phosphor.

We have discovered that a phosphor such as a copperactivated zincsulphide type phosphor, which will respond to ultraviolet or cathode rayexcitation but which will not electroluminesce, can be madeelectroluminescent by the addition of a substance such as lead. Such aphosphor can be made in two stages, that is, by adding the copper andfiring, then adding lead and refiring. A phosphor which has too littlelead initially, can be improved in brightness of light emission by theaddition of more 'lead and refiring.

The phosphors described above will generally have a blue or greenemission but the addition of manganese, either concurrently with thelead or copper, or afterward, will change the emission color to yellow.

We have discovered that an amount of lead in the final phosphor above alower limit of about 0.0002% by weight of the zinc sulphide providesgood electroluminescence, but if the amount is above about 0.02%, theluminescence will decrease.

The lead can be added to the copper-activated zinc sulfide in threeways: either (1) dry, that is, as a lead salt which will break down togive the oxide on firing; or (2) wet, that is, by suspending thephosphor in water, adding the lead as a soluble salt (for example aslead acetate) and then runing in a stream of H25 for a few minutes toprecipitate lead sulfide, after which the mixture is filtered and driedbefore refiring; or (3) moist, by wetting the phosphor with a solutionof a lead salt, such as the nitrate or acetate, and then drying themoistened material before retiring.

The amount of lead added to the mixture before firing will generallyneed to be between about 0.0002 to about 0.0012 mole per mole of zincsulfide, although the exact amount necessary will vary with the firingconditions, because the usual lead salts are volatile and there will beconsiderable loss during firing.

The effect of our lead treatment on a green phosphor is shown in TableI, the phosphor before treatment containing some lead but not enough forgood brightness, and being a zinc sulfide activated with about 0.001gramatom of copper for each mole of zinc sulfide.

Table I Before Treatment After Treatment Current (Microamps.)

Light Light Output Output Current (Microamps.

With no ZnO With ZnO lice The untreated green-emitting phosphor of theabove table had been washed in acetic acid, and had a fairly good lightoutput even before treatment. The addition of about 5% zinc oxideenhanced the effect of the lead, of which 0.001 mole were added in theform of lead carbonate for each mole of zinc sulfide in a dry process,the resultant mixture being then fired at about 1720 F. for aboutthree-quarters of an hour.

' The effect of .the wet method is shown in Table II below, where thezinc sulfide phosphor treated was a blue-electroluminescent phosphororiginally containing some lead, but not sufficient for good brightness.The phosphor contained 0.0006 gram-atom of copper per mole of zincsulfide. .An example of a blue-emitting phosphor is shown in a copendingapplication Serial No. 230,712, filed June 8, 1951, by Keith H. Butlerand Horace H. Homer.

In the case of the phosphor above, which had not been a'cid washed, itwas found unnecessary to add zinc oxide in order to gain the benefit ofthe lead treatment, the reason being that the lack of acid washing ofthe untreatedphosphor left some oxide in it. The amount of lead addedwas 0.0001 -mole per mole of zinc sulfide before firing.

The equivalence of the wet and dry treatments is shown in Table IIIbelow for a yellow-emitting zinc sulfide phosphor, containing 0.001gram-atom of copper and 0.005 gram-atom of manganese per mole of zincsulfide, which is a phosphor of the type shown in Keith H. Butlerscopending patent application Serial No. 230,711, filed June 8, 1951.

Table III Before Treatment After Treatment Light Current Light CurrentOutput (Mier0 Output (Microamps.) amps.)

Wet 25 115 Dry 25 110 210 Table IV Before Treatment After TreatmentLight Current Light Current Output (Miero- Output (Microamps amps.)

This phosphor was made in two stages. In the first stage, a copper saltwas added to a suitable quantity of Zinc sulfide, as shown for examplein copending application Serial No. 230,713, filed June 8, 1951, but thelead was omitted. The mixture was then fired for 40 minutes at atemperature of about 1720 F., in an essentially inert atmosphere, thatis with nitrogen flowing across the mixture. Since the mixture was firedin a furnace which had previously been used with lead-containingmixtures, there may have been a very small amount of residual lead inthe firing, so the phosphor gave a very faint electrolumineseence.

The product resulting from the first firing was then treated with lead,by the addition of about 0.0004 mole lead carbonate for each mole ofzinc sulfide, and refiring in the same sort of atmosphere at about 1720F. The luminescence rose from 2 to 460 units, an increase of 230 times.

The above was for a green-emitting phophor. If after the lead is addedand a green phosphor results, a manganese salt is added and the materialrefired, a yellow phosphor results. The amount of manganese added canbe, for example, 0.02 mole for each mole of zinc sulfide. Phosphorscontaining more manganese than this will generally be dark in color andof lower luminescence, and phosphors containing less than 0.002 mole foreach mole of zinc sulfide will have green or greenish-yellow emisslon.

Some lead should generally be added with the manganese, to make up forloss in lead during the manganese firing.

All the lead can be added with the manganese, if desired, instead ofbefore the addition of the manganese, or the manganese can be added withthe copper, and the lead added afterward. As a further variation of theprocess, the lead, or the lead and manganese, can be added first, andthe copper added afterward.

As a specific example of the preparation of a phosphor, we have mixedthe following ingredients as fine powders in the proportions shown:

Grams ZnO 80 ZnS 900 CuO 0.8

This mixture was then fired for about 40 minutes at about 1720 F. in afurnace with nitrogen flowing continuously over the mixture to providean inert atmosphere as described in a copending patent application,Serial No. 230,711, filed June 8, 1951, by Keith H. Butler.

The material after firing was substantially non-electroluminescentwithout further treatment. 5.3 grams of lead carbonate were then addedto the material as a fine powder and intimately mixed with it, afterwhich the resultant mixture was again fired for 40 minutes at about 1720F. with nitrogen flowing over the mixture to provide an inertatmosphere. The resultant material then electroluminesces strongly.While this firing method gives excellent results, other firing methodssuch as batch firing in a nitrogen atmosphere or even firing in acovered crucible in air may also be used in the production of thesephosphors.

The light output is given in the tables in arbitrary units proportionalto the brightness.

The method of lead treatment described herein is useful in various zincsulfide type phosphors, that is phosphors having zinc or cadmium, orboth, as the cations and sulfide or solenide, or both, as the anion.

The firing temperature for the lead treatment can vary from 1400 F. to1900 F., although I prefer temperatures in the neighborhood of about1720 F. In the final phosphor, the amount of lead can vary from about0.0002 to 0.02% by Weight, which is equivalent to 0.000001 to .0001gram-atom per mole of zinc sulfide.

The amount of copper can vary from about 0.0004 to about 0.0015gram-atom per mole of sulfide, and the manganese from zero to 0.025moles on the same basis.

The lead, copper and manganese activators may be added in any suitablecompound such as, for example, the carbonates, oxides, acetates,nitrates, and sulfates.

What We claim is:

1. The method of making an electroluminescent phosphor Which comprises:firing a mixture of Zinc sulfide with between about 0.0004 to about0.0015 gram atom of copper for each mole of zinc sulfide; then mixingthe resultant material with between about 0.0002 and about 0.0012 gramatom of lead per mole of zinc sulfide and a small amount of zinc oxide,and firing the mixture.

2. The method of making an electroluminescent phos phor Which comprises:firing a mixture of Zinc sulfide with between about 0.0004 to about0.0015 gram atom of copper for each mole of zinc sulfide; then mixingthe resultant material with between about 0.0002 and about 0.0012 gramatom of lead per mole of zinc sulfide and about 5% of zinc oxide andfiring the mixture. I

References Cited in the file of this patent UNITED STATES PATENTS2,447,322 Fonda Aug. 17, 1948 2,504,674 Fonda Apr. 18, 1950 2,624,857Mager Jan. 6, 1953 2,745,811 Butler May 15, 1956 FOREIGN PATENTS 873,860France July 22, 1942

1. THE METHOD OF MAKING AN ELECTROLUMINESCENT PHOSPHOR WHICH COMPRISE:FIRING A MIXTURE OF ZINC SULFIDE WITH BETWEEN ABOUT 0.0004 TO ABOUT0.0015 GRAM ATOM OF COPPER FOR EACH MOLE OF ZINC SULFIDE; THEN MIXINGTHE RESULTANT MATERIAL WITH BETWEEN ABOUT 0.0002 AND ABOUT 0.0012 GRAMATOM OF LEAD PER MOLE OF ZINC SULFIDE AND A SMALL AMOUNT OF ZINC OXIDE,AND FIRING THE MIXTURE.